"Indeed, just how sensitive markets remain to the Syrian situation was shown when U.S. House Speaker John Boehner spoke in support of military action in Syria yesterday," wrote Deutsche Bank analyst Jim Reid in a market report. "This was enough to drive markets lower into the European close and sent the S&P 500 (+0.42% on the day) down 9 points (0.5%) within a matter of minutes."

Obama is seeking approval from Congress to carry out a military strike after saying U.S. intelligence showed that Syria used chemical weapons which killed more than 1,400 people.

Several key Democrats and Republicans have thrown their support behind Obama's plan in recent days, though a number of hurdles must still be overcome before the U.S. can take military action.